The mixture consisting of diphenylmethane diisocyanate (hereinafter referred to as "MDI") and polymethylene polyphenyl isocyanate (hereinafter referred to as "PMPPI"), a higher homolog of MDI, is popularly called "crude MDI" and is mass-produced on a commercial scale as an important raw material for polyurethanes.
The aforementioned dinuclear substance, MDI, is mainly composed of 4,4'-diphenylmethane diisocyanate (namely, pure MDI)and is generally isolated from crude MDI by distillation. In recent years, the demand for this dinuclear MDI as the raw material such as for polyurethane elastomer, Spandex, synthetic leather coating agent, and reaction injection molding polyurethane is rapidly increasing. Accordingly, the desirability of developing a method capable of economically producing the crude MDI containing the dinuclear MDI in a high concentration and entrainning other isomers than the 4,4'-isomer in a relatively low concentration has been finding growing recognition.
For the production of this crude MDI, for example, there has been adopted the method which obtains the desired crude MDI by causing condensation of aniline with formaldehyde in the presence of an acid catalyst thereby forming a mixture consisting of diphenylmethane diamine and polymethylene polyphenylamine (hereinafter referred to as "crude MDA"), subsequently allowing phosgene to react upon the crude MDA in a solvent thereby deriving a corresponding carbamic acid choride, then converting the carbamic acid chloride through thermal decomposition into crude MDI and hydrogen chloride, and expelling from the resultant reaction mixture both hydrogen chloride and the reaction solvent.
For the production of crude MDA containing dinuclear diphenylmethane diamine (hereinafter referred to as "MDA") in a high concentration, this conventional method must rely on the use of aniline and an acid catalyst such as hydrochloric acid both in large excesses relative to formaldehyde. Consequently, the amount of base which must be used for the purpose of neutralization is inevitably large. Besides, the recovery of the unaltered aniline is costly. Thus, the method proves to be uneconomical. Moreover, this method has the disadvantage that in spite of an increase in the proportion of the dinuclear component in the crude MDA, the proportion of the 4,4'-isomer in the dinuclear component decreases and the proportions of the rather unwanted isomers, i.e. 2,4'-MDA and 2,2'-MDA, are increased.
Further, this conventional method is disadvantageous in that the process involved necessitates the use of violently poisonous phosgene, that the use of phosgene entails generation of corrosive hydrogen chloride in a large volume, that the final product entrains hydrolyzable chlorine compounds, and that these by-productss are extremely difficult to separate away. For the purpose of eliminating all these drawbacks, research is under way in search of new processes capable of producing MDI without the use of phosgene.
As one solution for the elimination of the use of phosgene, for example, the method which comprises causing condensation of N-phenylcarbamate with formaldehyde thereby giving rise to a mixture consisting of diphenylmethane dicarbamate and polymethylene polyphenylcarbamate, a higher homolog of diphenylmethane dicarbamate,, ad subsequently subjecting this mixture to thermal decomposition has been proposed (U.S. Pat. Nos. 4,349,484 and 4,307,029 and European Patent Nos. 28,337 and 30,039).
These methods under development for the aforementioned purpose, however, are such that the proportions of the dinuclear MDI in the produced crude MDI fall roughly in a low range of 40 to 78%, the range virtually comparable with the range usual with the process using phosgene. Thus, these methods are not satisfactory.
Various other methods have been proposed which produce condensation mixtures consisting of diphenylmethane dicarbamates and their higher homologs by the condensation of N-phenylcarbamates with formaldehydes. For example, the method which resorts to the reaction of N-phenylcarbamates with condensing agents such as formaldehyde, paraformaldehyde, methylal, and trioxane in the presence of various acids such as mineral acids and organic sulfonic acids. If relatively severe conditions are used in this reaction, for example, if a strong acid is used in a large amount, the reaction temperature is high or the reaction period is extended, not only is the desired diphenylmethane dicarbamate produced but also polynuclear polymethylene polypenylcarbamates having the following formula are produced in a significant amount: ##STR1## (wherein R is an alkyl group, aromatic group or an alicyclic group; z is an integer of 1 or more). Furthermore, if a strong liquid acid is used, much difficulty and hence a lot of cost is entailed in separating the acid from the reaction mixture and recovering the same in a reusable form.
In order to eliminate this defect with the recovery of acids, a method was proposed for using an aqueous acid solution having a concentration of 10% or higher (British Patent No. 2,044,252, Japanese Patent (OPI) Nos. 81850/80 and 81851/80 and Chemical Abstracts 93 169057e). This method is effective for acid recovery because as shown in the working examples, if aqueous acid solutions having a concentration of not more than 50% are used, the acid can fairly easily be separated from the organic phase in the form of layers. However, the presence of a great amount of water renders it difficult to complete the reaction without leaving a significant amount of compounds having a methylene-amino bond(--CH.sub.2 --N&lt;) wherein the methylene group is bonded to the nitrogen atom in the carbamate group. In order to complete the reaction without these compounds, less water must be used to increase the acid concentration to, for example, 80% or higher. However, this causes the hydrolysis of the starting compound or the reaction product, or leaves them to dissolve in the concentrated aqueous acid solution in a large quantity, and as a result, the separation of the product from the acid solution becomes difficult.
In any event, it is not industrially advantageous to carry out a one-step condensation of N-phenylcarbamates with an aqueous solution of acid and to use the resulting product in the preparation of isocyanates. More specifically, dinuclear, trinuclear or other polynuclear compounds having the methylene-amino bond cannot be easily separated from the condensation product containing diphenylmethane dicarbamates and polymethylene polyphenylcarbamates. If the condensation product containing these compounds with the methylene-amino bond is decomposed thermally, these compounds do not provide the desired isocyanates. Furthermore, they enter into various side reactions with the isocyanates derived from the carbamates such as diphenylmethane dicarbamates, and in consequence, the yields of the desired isocyanates are reduced. In addition, the resulting by-products cannot be easily separated from the desired isocyanates, particularly, the polynuclear polymethylene polyphenyl isocyanates, and they are in all cases present in the final product generally referred to as a polymeric isocyanate, and properties of the product are impaired.
It is therefore necessary to perform the condensation of N-phenylcarbamates in such a manner that a minimum amount of the compounds with the methylene-amino bond is left in the condensation product. One method that has been proposed for attaining this object is described in U.S. Pat. No. 4,146,727, wherein these compounds with the methylene-amino bond are subjected to a rearrangement reaction, under substantially anhydrous conditions, with a protonic acid catalyst having a strength of at least the magnitude of a 75% sulfuric acid, or a Lewis acid at a temperature of 50.degree. to 170.degree. C. so as to rearrange the methylene group, which was bonded to the nitrogen atom, to bond to the benzene ring. However, this method must use a large amount of concentrated sulfuric acid or paratoluenesulfonic acid and again requires complicated procedures and great cost for separating and recovering these acids from the reaction mixture.
Japanese Patent (OPI) No. 7749/81 and Chemical Abstracts, 94 209480s propose a method for producing polymethylene polyphenylcarbamate by heating only bis(N-carboalkoxyanilino)methane, which is a compound having the methylene-amino bond, in the presence of an acid catalyst. However, this method is not ideal for selective production of the diphenylmethane dicarbamate because it causes not only the desired rearrangement reaction but also the undesired condensation reaction, and trinuclear and other polynuclear polymethylene polyphenylcarbamates are formed as by-products in addition to the desired diphenylmethane dicarbamate. Furthermore, the reaction is slow and the rearrangement reaction is not completed without leaving the residual bis(N-carboalkoxyanilino)methane in the reaction product.
U.S. Pat. No. 4,319,018, British Patent No. 2,054,584, Japanese Patent (OPI) No. 12357/81 and Chemical Abstracts, 94, 124715t propose a method for producing diphenylmethane dicarbamates and polymethylene polyphenylcarbamates by reacting N-phenylcarbamates with formaldehyde or its precursor in the presence of both an acid catalyst and the compounds having the methylene-amino bond. However, this method is unable to reduce the content of the compounds with the methylene-amino bond, and the compounds are unavoidably left in the condensation product in an amount as much as ten-odd percent by weight.